1. Field of the Invention:
The present invention relates to a filtering system for separating two signals of different frequencies or different frequency bands in a composite signal, one of the signal to be separated being in a relatively low frequency band (e.g., from D.C. to 300 MHz) and the other signal being in a relatively high frequency band (e.g. from 800 Mhz to 20 GHz). The invention also relates to a circuit for installation of this circuit on a wall of a resonant cavity.
2. Description of the Related Art:
Ordinarily, in the telecommunication field, it is convenient to transmit simultaneously, e.g. on a same carrier, such as a cable, two or more signal types, e.g. a high frequency signal that must pass through a pass-band filter and a low frequency signal, that can also include a D.C. level.
For example, FIG. 1a shows the frequency bands S2, e.g. from 0 to about 300 Mhz, and S1, e.g. from 925 to 970 Mhz.
FIG. 1b is a block diagram schematic of a conventional circuit, in which the composite signal S1+S2 is applied to two filters connected in parallel F1A (that allow only band S1 to pass), and F2B, that allows only band S2 to pass.
The technical literature of the prior art discloses many possible circuits to solve this problem. However, these known circuits have many drawbacks, arising in particular from losses, distortion of the bands (especially the high frequency bands), size, cost, etc.
The filtering scheme of FIG. 1b is typically carried out in an embodiment utilizing a low-pass filter F2B with an inductive input, for example the configuration of FIG. 2, in which S1+S2 is applied on the node N that has, at its left side, filter F1A, including substantially a high capacitor C followed by an inductance LH (connected to ground) and, at its right side filter F2B including the inductances L1b, L2b, L3b connected in series and capacitances C1b, C2b, etc. connected in parallel to ground.
The capacitor C prevents the D.C. and low frequencies of S2 from being short-circuited to ground through the input coupling inductance of pass-band filter F1A. The pass-band filter F2B transmits the D.C. and low frequencies.
Even if the above-described prior art circuit can solve the "size" problem, it has many drawbacks, including:
high input capacitance, caused by the sum of C with the capacitances C1b, C2b, etc. of the low-pass filter F2B;
high insertion loss in the high frequency band S1, caused by the fact that the low-pass filter F2B is upstream of, i.e. precedes, the pass-band filter F1A;
the capacitance inserted in series with the input inductance LH of low-pass filter F1A causes a resonance that creates an undesirable attenuation in a band near the bands of the input signal.